NEW ORLEANS (AP) — Paper producer Temple-Inland Inc. took responsibility Wednesday for a huge fish kill on the Pearl River that borders Louisiana and Mississippi, blaming a discharge from one of its paper mills.

In a statement, Austin, Texas-based Temple-Inland apologized "for the impact this issue at the mill has had on the Pearl River." The company said it is working to remove the hundreds of thousands of dead fish and mussels and restore the river's quality.

Temple-Inland said it shut down its Bogalusa, La., plant immediately on Saturday after tests showed the plant would exceed its allowable output of discharge into the river. The company said it immediately informed Louisiana environmental officials of the problem and began working to restore the river's water quality.

"We never lose sight that we are members and supporters of the Bogalusa area and apologize for the impact this issue at the mill has had on the Pearl River, its aqua culture and surrounding communities," Temple-Inland chief executive Doyle Simmons said. "We are working diligently and expeditiously to remove the fish kill and restoring the quality of the river."

Jay Wilson, the company's vice president of environment, safety and quality, said the discharge consisted mostly of water and residue of a substance known as "black liquor." Black liquor is a byproduct of the process in which wood is converted to pulp. Most of the black liquor produced in Bogalusa is used by the plant as an alternative fuel to generate steam, Wilson said.

Wilson said black liquor is biodegradable. The problem with the river was not due to any toxicity, but the concentration of black liquor, which drained off the water oxygen supply following an extended period of hot temperatures, he said.

The Mississippi Department of Environmental Quality said the kill stretched 35 miles to 40 miles along the river from Bogalusa to the Mississippi Sound.

Temple-Inland hired an environmental cleanup company and, on Wednesday, had 30 boats and about 100 people working to collect dead fish, which will be buried in a permitted landfill, Wilson said. The company was working with local, state and federal officials on the project, he said. The mill remained closed Wednesday.

In Mississippi, authorities took action to bring fresh water into the river — and alleviate additional fish kills — by temporarily increasing discharges from the Ross Barnett Reservoir near Jackson. The action late Tuesday by the Pearl River Valley Water Supply District also was designed to dilute pollutants.

St. Tammany Parish and the Louisiana Governor's Office of Homeland Security and Emergency Preparedness set up a command post Wednesday on the West Pearl River. Parish President Kevin Davis said the environmental cleanup company would be hiring area residents to help with the cleanup. Interested citizens should report to the Mobile Command Center beginning at 5 p.m. Wednesday.

Wilson said he expected the cleanup to take several days.

On Wednesday, the Lake Pontchartrain Basin Foundation said at least some pollution had entered the lake. The foundation said that on Tuesday, a 10-mile long line of white foam stretched from Rigolets Pass to Bayou Banfouca, scattering dead catfish. However, the foundation said oxygen levels in the lake appeared normal.

"It is likely the foam and dead fish were carried by tides in normal lake water and may not indicate that significant polluted water has entered Lake Pontchartrain," the foundation said in a statement.

Scientists blamed the fish kill on a lack of water oxygen stemming from the pollution. On Wednesday, the Louisiana Department of Wildlife and Fisheries said tests showed oxygen levels were returning to normal on the river.

The lake foundation's acting executive director, John Lopez, said the oxygen level of the river would recover, but it likely would take a while for affected species to relocate along the river and repopulate. He said there were unknowns to the long-term effect of black liquor, since the composition of the substance varies from plant to plant.

It likely will take several days for the additional discharge at the Ross Barnett Reservoir to reach the affected area of the river, said Trudy Fisher, executive director of the Mississippi Department of Environmental Quality.

The reservoir was created by impounding the Pearl River between Madison and Rankin counties in Mississippi. It was completed in 1965 and has since become a popular place for boating, sailing, camping and fishing.

John Sigman, the water supply district's general manager, said the increased discharges would have little effect on the reservoir, other than a slight decrease in the water level.

"We will return to normal discharge rates after we are able to determine the effects on the lower Pearl River," Sigman said.

The Louisiana fisheries agency said 24 species had been identified as part of the kill, including paddlefish, American eels, catfish, bass, bluegill and shad, along with two species of freshwater mussels and the endangered Gulf sturgeon. The Mississippi DEQ said the spill also threatened to harm the endangered Ringed Sawback Turtle and an endangered mussel.